I’ve managed to fix this bug!
Normally, you cannot kill Louis, as he’s marked as a “special” character.
However, I read on UESP wiki that if you see an Orc killing Louis in the wild, the ones outside Whiterun Stables become mortal and can be killed.
So I tried it and it worked! Some time ago I remember the orc killing him in the wild. So I killed one of the 8 Louis from afar with a bow, and the other 7 attacked me. After wiping them out and looting their bodies, I moved away for a few in-game days.
After returning to Whiterun Stables, all 8 of the bodies have vanished and none have respawned, yet.
Result!